This invention relates to light-emitting diodes and, more particularly, to light-emitting diodes that are capable of emitting two separate wavelengths.
The very wide transmission window from about 0.8 microns to 1.6 microns in present low-loss optical fibers permits the use of wavelength multiplexing to increase the transmission capacity of a single fiber. Dual-wavelength photodetectors that are capable of operating within this wavelength range have already been disclosed in the prior art. See, for example, the article entitled "Dual Wavelength Demultiplexing InGaAsP Photodiode" by J. C. Campbell et al, Applied Physics Letters, Vol. 34, No. 6, March 15, 1979, pp. 401-402. Independent light-emitting diodes have already been proposed for use in connection with this two wavelength demultiplexing photodetector in order to provide transmission systems having greater information carrying capacity.
A single device, dual-wavelength light emitting diode has been disclosed in the prior art in the article entitled "A New InGaAsP/InP Dual-Wavelength LED" by S. Sakai et al., Applied Physics Letters, Vol. 35, No. 8, Oct. 15, 1979, pp. 588-589. In this Sakai et al dual wavelength LED, five layers including two quaternary layers are epitaxially grown on an indium phosphide substrate and approximately one half of the device is removed by chemical etching in order to permit a direct electrical connection to the bottom quaternary layer. The resulting device shown in FIG. 1 of the article presents an irregular top surface that cannot be readily attached to any heat sink material. The device shown in the Sakai et al article was attached to a copper heat sink, but as pointed out in the article, the output of the light-emitting junction in the uppermost quaternary layer began to saturate at 40 milliamps because of the high resistance and poor heat sink. Although it is not stated, this heat sink was very likely bonded to the substrate surface that is opposite to that of the epitaxial layers. As implied by this article, a higher output could be obtained from the light-emitting junction if the device had better heat sinking.